ArticlePDF Available

A palaeogravity calculation for the gigantic fossil arthropod Hibbertopterus (Eurypterida)

Authors:

Abstract and Figures

One fundamental technique to quantify palaeogravity is to compare dynamically similar extinct and extant animals. This technique is applied to a fossil trackway of the gigantic fossil arthropod Hibbertopterus (Eurypterida). The results indicate a tentative palaeogravity of about 0.37g for 330 Ma.
Content may be subject to copyright.
Martin Whyte (2005) reported the discovery of a
complex fossil trackway in Carboniferous age fossil
deposits in Scotland (see figure 1). It was made
roughly 330 million years ago by a huge, six-legged
water scorpion that was about 1.6 m long and a me-
tre wide. This was believed to be the largest terrestri-
al trackway of a walking arthropod found so far (0.2
m wider than any other trackway of this type), as
well as the first record of locomotion on land for a
species of Hibbertopterus (Eurypterida). This evidence
of lumbering movement indicated that these giant
arthropods, now extinct, could survive out of water
at a time when the earliest tetrapods were making
their transition to the land. The trackway was ex-
posed on a bedding plane close to the base of a sand-
stone section in a non-marine sequence. It is 6 m
long, 0.90–0.98 m wide and consists of sinuous,
paired belts of appendage prints flanking a sub-cen-
tral groove. The animal appeared to be slowly crawl-
ing across the ground, while in places the footprints
have been erased by the central groove, which was
made by the posterior part of the animal being
dragged behind it. An occasional oblique lineation
on the sides and base of the groove indicated that
the motion was jerky. The slow, stilted progression,
together with the dragging of the posterior, indicat-
ed that the animal was not buoyant but moving
across land. The pattern and character of the limb
prints was identified as being most consistent with
the relatively short-limbed Hibbertopterus. The track-
way maker would have been comparable in size to
the largest known hibbertopteroid body fossils,
which have head shields 0.65 m wide.
The mechanical stress imposed on such a large ar-
thropod seems to imply that it would struggle to
walk on land. As size increases, the volume and
weight of an arthropod would be cubed while the
leg area and strength would only be squared. This
means the arthropod must reach a maximum size
limit when its legs wouldn’t be able to support its
mass. In order to try to assess if it was possible for
such large arthropods to walk on land Dalingwater
(1985) used the extant Limulus, a horseshoe crab, as
Abstract
One fundamental technique to quantify palaeogravity is to compare dynamically similar extinct and extant
animals. This technique is applied to a fossil trackway of the gigantic fossil arthropod Hibbertopterus
(Eurypterida). The results indicate a tentative palaeogravity of about 0.37g for 330 Ma.
Key words: Palaeogravity, Hibbertopterus (Eurypterida).
A palaeogravity calculation for the gigantic fossil arthropod
Hibbertopterus (Eurypterida)
Stephen W. Hurrell
email: papers@dinox.org
First Published: 15th October 2021
Cite: Hurrell, S.W. (2021). A palaeogravity calculation for the gigantic fossil arthropod Hibbertopterus
(Eurypterida).http://dinox.org/hurrell2021a
OPEN ACCESS Freely available online www.dinox.org
2
Hurrell, S.W. (2021). A palaeogravity calculation for the gigantic fossil arthropod Hibbertopterus (Eurypterida)
www.dinox.org
Figure 1. Photograph of a track identified as a short-limbed Hibbertopterus (Eurypterida) by Whyte (2005),
complete with an enlarged section. The track is the long curved line stretching from left to right, just above
the geological hammer, with additional marks on the two sides of the curved line. Note that the sandstone
bed is dipping at 45 degrees away from the viewer. The original photograph is available from Wikipedia
Commons (see reference).
Figure 2. A modern day horseshoe crab trackway, with an enlarged section. Note the claw markings, tail
drag and wandering path and its similarity with the Hibbertopterus (Eurypterida) trackway. The original
photograph is available from Wikipedia Commons (see reference).
3
Hurrell, S.W. (2021). A palaeogravity calculation for the gigantic fossil arthropod Hibbertopterus (Eurypterida)
www.dinox.org
a model. These arthropods are the closest living rel-
atives to the extinct water scorpions. Although
horseshoe crabs are primarily aquatic animals they
are also capable of excursions onto land. Calcula-
tions based on them implied that, if the animal
maintained dynamic similarity, the cuticles of the
legs would become too thick to leave sufficient space
for muscles within the limbs of a large animal. Thus
the horseshoe crab is about as large as we might ex-
pect such an animal to grow. Briggs et al (1991) not-
ed that larger arthropods would need some method
to offset potentially unacceptable stresses on their
limbs, for example by changes in shape or in skele-
tal materials, or by a reduction in locomotor per-
formance. On theoretical grounds, an arthropod
with a body length of around 1.6 m would not be
able to move about as freely as this fossil trackway
indicates.
The large arthropods would be able to move freely
on land if palaeogravity was less at that time. Assum-
ing that the Hibbertopterus recorded in the fossil
record moved in a dynamically similar way to an
extant Limulus we can calculate palaeogravity from:
g = Sl / Sh
where Sl is the relative size of the Limulus, and Sh is
the relative size of Hibbertopterus. The derivation of
this formula is discussed in detail in Hurrell (2011).
In order to use this formula we need to reassure
ourselves that these two animals were moving in
dynamically similar ways. The track identified as a
short-limbed Hibbertopterus (see figure 1) can be com-
pared with a typical track left by a giant horseshoe
crab (see figure 2). The claw markings, tail drag and
wandering path are similar for both trackways so
they did appear to be moving in dynamically similar
ways.
Assuming that the Hibbertopterus reached its maxi-
mum size limit in a reduced gravity, it can be com-
pared with the largest horseshoe crab in a 1g
environment. Mainichi (2005) reported finding a
giant horseshoe crab measuring 0.795 m from head
to tail, with a mass of five kilograms, in the sea near
the Kujukushima islands off Sasebo, Nagasaki Pre-
fecture of Japan. The horseshoe crab's shell meas-
ured 0.353 m in width. It was handed over to the
Saikai Pearl Sea Center aquarium, which conducts
studies of the Kujukushima islands. The previous
largest giant horseshoe crab confirmed in Japan
measured 0.63 m in length and weighed about 3.5
kilograms. The five kilogram size has been taken as
the largest size of extant horseshoe crabs.
The horseshoe crab has a large protective shield cov-
ering most of its legs and head. If this protective
shield is ignored it would be very similar to an an-
1
02
metre
Figure 3. The Hibbertopterus
(Eurypterida) drawn to scale (on the
left) compared with the calculated size
of a horseshoe crab in a palaeogravity
of 0.37g (on the right), assuming the
horseshoe crab continued to have the
same dynamic capabilities. Both
trackways are 0.95 m wide. The
horseshoe crab has a large protective
shield covering most of its legs and
head. If this shield is ignored its legs
and body would be similar to an ancient
sea scorpion and it would leave similar
trackways. The Hibbertopterus is
redrawn after Tetlie (2007); the
horseshoe crab is redrawn after Holder
(1892).
4
Hurrell, S.W. (2021). A palaeogravity calculation for the gigantic fossil arthropod Hibbertopterus (Eurypterida)
www.dinox.org
cient sea scorpion. Compared with the ancient sea
scorpion, its legs appear to move in a similar man-
ner, leaving similar footprints whilst dragging its
posterior across the ground. Theoretically, a simple
comparison of the widths of the two trackways
would provide the two relative sizes needed to calcu-
late palaeogravity. The width of the largest horse-
shoe crab trackway would be slightly less than the
width of its protective shield, at 0.345 m, while the
width of the ancient Hibbertopterus trackway is about
0.94 m.
Thus:
g0.345 / 0.94 0.37g
This estimate of palaeogravity compares an ancient
sea scorpion to a dynamically similar horseshoe crab
and so it is probably more accurate than previous
palaeogravity estimates for sea scorpions, as given in
Hurrell (2012), which compared them to extant
scorpions. It would be better to use the new esti-
mate of about 0.37g instead of the older estimate.
Another way to envisage the comparison of dynami-
cally similar extinct and extant arthropods is to scale
up the extant horseshoe crab to the size if would be
in a reduced gravity of 0.37g, assuming it keeps the
same dynamic capabilities as the 1g animal. The two
trackways would be the same size, as represented in
figure 3.
References
Briggs, D. E. G., Dalingwater, J. E., & Selden, P. A.
(1991). Biomechanics of locomotion in fossil arthro-
pods. In Biomechanics and evolution edited by J. M.
V. Rayner and R. J. Wootton, Cambridge University
Press 1991, (pp. 37-56).
Dalingwater, J. E. (1985). Biomechanical approach-
es to eurypterid cuticles and chelicerate
exoskeletons. Earth and Environmental Science Trans-
actions of The Royal Society of Edinburgh,76(2-3), 359-
364.
Holder, C. F. (1892). Along the Florida reef. D. Ap-
pleton.
Hurrell, S. W. (2011). Dinosaurs and the Expanding
Earth. Oneoff Publishing.com
Hurrell, S. (2012). Ancient Life’s Gravity and its Im-
plications for the Expanding Earth. In The Earth
Expansion Evidence–A Challenge for Geology, Geophysics
and Astronomy-Selected Contributions to the Interdiscipli-
nary Workshop of the 37th International School of Geo-
physics. Aracne Editrice, Roma.
Mainichi (2005). Giant horseshoe crab found off Na-
gasaki Prefecture. Pub: November 22, 2005.
Tetlie, O. E. (2007). Distribution and dispersal histo-
ry of Eurypterida (Chelicerata). Palaeogeography,
Palaeoclimatology, Palaeoecology, 252(3-4), 557-574.
Whyte, M. A. (2005). A gigantic fossil arthropod
trackway. Nature,438(7068), 576-576.
Wikipedia Commons is a collection of mostly freely
reusable media files to which anyone can contribute.
See site for any restrictions on reuse. The Hibber-
topteroid track can be found at:
https://commons.wikimedia.org/wiki/File:Hibbertopt
eroid_track.jpg The horseshoe crab track can be
found at:
https://commons.wikimedia.org/wiki/File:Horseshoe
_Crab_Tracks_(7644537456).jpg
ResearchGate has not been able to resolve any citations for this publication.
Conference Paper
See the full text in: Extended Abstracts of the 37TH INTERDISCIPLINARY WORKSHOP "THE EARTH EXPANSION EVIDENCE: A challenge for Geology, Geophysics and Astronomy" October 2011 DOI: 10.13140/2.1.1522.4643
Article
Microstructural features of eurypterid cuticles are analysed from a biomechanical viewpoint: some fibrous elements are now considered to resemble the macrofibres of extant arthropod cuticles; possible preferred orientation zones in Mycterops are related to directional stresses; pore canals are not viewed as acting as crack-stoppers but laminae (sensu Dennell 1978) may have served this function. Could some eurypterids have walked on land? - this problem is approahced by using extant Limulus as a model. It leads on to the use of scaling exponents to determine the limits that possessing an exoskeleton places on the size of land arthropods: moulting may be the limiting factor. Possble critical factors limiting the size of aquatic arthropods are discussed briefly. -Author
Article
The distribution and dispersal histories of Eurypterida are examined in a phylogenetic and palaeogeographic context. It is suggested that all the eurypterid clades originated and had most of their evolutionary history on the palaeocontinents of Laurentia, Baltica, Avalonia, the Rheno–Hercynian Terrane and some presence in Siberia. The Siluro–Devonian Stylonurina and the eurypterine superfamilies Eurypteroidea and Waeringopteroidea are confined in their entirety to these continents. The Late Palaeozoic records of Adelophthalmoidea and Hibbertopteroidea in China, South America and Africa can be explained by ‘rafting’ on the continental plates that amalgamated into the supercontinent Pangaea. However, two clades have distributions that suggest abilities to cross open stretches of ocean; the Silurian and Devonian pterygotoids and some Devonian adelophthalmoids. However, one Silurian mixopteroid and the basal swimming form Onychopterella outside the ‘core-area’ suggest the dispersal patterns are more complicated. The pterygotoids might be the only clade whose members frequently undertook oceanic excursions. Since almost all eurypterid evolution took place in Laurentia, Baltica and Avalonia (and some in the Early Devonian of the Rheno–Hercynian Terrane) prior to the Carboniferous, it is unlikely that many taxonomically rich pre-Carboniferous eurypterid faunas will be encountered outside of these palaeocontinents.
Article
A unique, complex trackway has been discovered in Scotland: it was made roughly 330 million years ago by a huge, six-legged water scorpion that was about 1.6 m long and a metre wide. To my knowledge, this is not only the largest terrestrial trackway of a walking arthropod to be found so far, but is also the first record of locomotion on land for a species of Hibbertopterus (Eurypterida). This evidence of lumbering movement indicates that these giant arthropods, now extinct, could survive out of water at a time when the earliest tetrapods were making their transition to the land.
Along the Florida reef
  • C F Holder
Holder, C. F. (1892). Along the Florida reef. D. Appleton.
Dinosaurs and the Expanding Earth
  • S W Hurrell
Hurrell, S. W. (2011). Dinosaurs and the Expanding Earth. Oneoff Publishing.com
Giant horseshoe crab found off Nagasaki Prefecture
  • Mainichi
Mainichi (2005). Giant horseshoe crab found off Nagasaki Prefecture. Pub: November 22, 2005.